when do you need to enable 'SSI' on a web site? i am not sure what that is, what it is used for, and what does it do?

SSI = Server Side Includes. Server side includes are used for server-parsed html. In other words, the web server will parse through all .shtml files looking for executable commands.

A lot of counters, poll scripts, and header/footer files use SSI. It does an a load to your server, but unless it is a large site, that load is negligable.

For more information look here:

http://hoohoo.ncsa.uiuc.edu/docs/tutorials/includes.html